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OUR PHYSICAL WORLD 



exact, the measurement should be made from the focus to the 

 optical center of the lens, but the rough measurement to the 

 face of the lens is adequate for our purpose. 



Set a lighted candle on the table. Hold the lens in your 

 left hand a foot from the candle flame. Hold a sheet of paper 

 in your right hand on the opposite side of the lens from the 

 candle flame, and move this sheet closer to, or farther from, 

 the lens until a clear image of the candle flame is seen on the paper. 

 Note the size of the image. Move the lens to about 6 inches from 

 the flame. Note now that the image is no longer distinct. To 

 obtain a distinct image the screen must be moved farther away 



FIG. 143. Candle and screen are at the conjugate foci of the lens. Two pencils 

 of light are shown, focusing to form two points of the image; similar pencils ema- 

 nate from other points of the candle, and are brought to a focus to form correspond- 

 ing points of the image. 



from the lens, and the image will be much larger than before. 

 On the other hand, if the lens is moved so that it is 2 feet 

 from the candle flame, the screen must be brought nearer the 

 lens, and the image will be smaller than in either previous position 

 of the lens. It is evident that the light emanating from the 

 candle flame is brought to a focus at the place where the image is 

 formed. If the candle and the screen were interchanged in 

 position, there would still be a sharp image of the flame upon 

 the screen. These two points are known as conjugate foci, and 

 the nearer one of these is to the lens, the farther away the other 

 must be (Fig. 143). 



Lenses may be either convex or concave, the former bringing 

 parallel rays of light to a focus, the latter making such rays 



